As digital household appliances have become common, there is an increasing need for higher-speed and higher-performance electric and electronic devices. In electric and electronic devices, semiconductor elements for electronic control such as LSIs and CPUs consume more power and thus produce more heat because of higher integration and higher-speed operations in computers. Heat needs to be dissipated from such semiconductor elements to prevent problems such as the occurrence of failures in the semiconductor elements. A method of dissipating heat for general electric and electronic devices is to install a cooling part such as a heatsink in the devices and forcibly cool the heatsink by using a cooling fan or the like. In compact electric devices such as laptop computers and densely-packed electronic devices, heat is dissipated by means of application of silicone grease because of limitations such as the small space available for the installation of a cooling fan or the like. However, silicone grease have problems such as low work efficiency, part contamination due to the squeezing-out of the grease after application, and limited use under high load due to poor cushioning.
Thermally conductive sheets are used to meet the requirements for higher-performance electric and electronic devices. A thermally conductive sheet is a flexible sheet which is effective in placing it between a rigid cooling part such as a heatsink and a flexible heating element and improving the proximity of both parts. If both parts are brought closer to each other, heat can be conducted to the cooling part more efficiently.
The most commonly used thermally conductive sheet is a thermally conductive sheet produced by mixing a filler having a relatively high thermal conductivity into a silicone rubber. This silicone rubber-based thermally conductive sheet is easy to handle. However, the silicone rubber-based thermally conductive sheet has problems such as expensive silicone resin itself as a raw material and an increased number of steps due to a required curing step. In addition, a silicone resin contains low molecular weight siloxane in it, so when this thermally conductive sheet is placed on a heating element for use, low molecular weight siloxane gas is generated. The gas may adhere to an electrode contact or the like to generate silicon dioxide, leading to contact failure.
In addition, a thermally conductive sheet is also required to meet various physical characteristics other than thermal conductivity.
For example, a thermally conductive sheet is required to be electrically insulating to prevent failure due to the passage of current through an electronic substrate. This is because the sheet is often placed on the electronic substrate with the sheet in contact with the electronic substrate.
In addition, if there is a gap between a heating element such as a CPU and a cooling part such as a heatsink, sometimes a thermally conductive sheet having a thickness of greater than 1 mm and 3 cm or less is used. In this case, the thermally conductive sheet is required to be flexible and tough. This requirement is intended to prevent material breaking when the sheet is fixed with the thermally conductive sheet under pressure between the heating element and the cooling part.
In addition, when a thermally conductive sheet having a thickness of 1 mm or less is placed on, for example, a CPU on an electronic substrate, sometimes sheet positioning fails on the first try and is separated to place it again. In this case, if the sheet has poor toughness, the sheet itself is torn off, resulting in poor yield.
In addition, thermally conductive sheets are used not only in electric and electronic devices but also for house floor heating. This latter use is intended to conduct heat from hot water flowing through circulation pipes to the floor. A thin aluminum film is used as a thermally conductive sheet at present. However, the aluminum film lacks cushioning, so it provides poor proximity of the pipes and the floor and cannot heat the whole floor efficiently and uniformly.
Furthermore, if a thermally conductive sheet is used for internal parts of low power electric and electronic devices, the sheet is required to be flame-retardant in view of safety.
Patent Documents 1 and 2 proposes that a zinc oxide whisker is added to a resin. This addition is intended to make the resin composition electrically conductive or make the resin mechanically stronger.
Patent Document 3 proposes that a filler is added to a styrene-based hydrogenated copolymer produced by hydrogenating a copolymer comprising a conjugated diene and a vinyl aromatic. This addition is intended to make the resin composition more resistant to wear and abrasion and mechanically stronger. Although this patent document describes a spherical zinc oxide as a filler, the use of the spherical zinc oxide does not allow excellent thermal conductivity to develop. In addition, the patent document has no description of flame retardancy.
Patent Document 4 proposes a resin composition produced by mixing a paraffin oil, a thermally conductive filler, and a flame retardant into a mixture of a styrene-based thermoplastic elastomer and a propylene-based polymer. The resin composition uses the propylene-based polymer for higher processability and heat resistance and the paraffin oil for use of a large amount of the filler and flexibility. The styrene-based thermoplastic elastomer and the propylene-based polymer are incompatible with each other regardless of whether or not the paraffin oil is added. For this reason, disadvantages of the resin composition are poor toughness as a material and brittleness occurring when it is processed into a sheet or a molded body. In addition, the amount of the paraffin oil is very large because it is 3.5 times or more the total amount of styrene-based thermoplastic elastomer and propylene-based polymer, so the paraffin oil easily bleed out of the interface between both phases of the incompatible styrene-based thermoplastic elastomer and propylene-based polymer. For example, when a thermally conductive sheet comprising the composition is placed on a CPU, the operation of the CPU allows the thermally conductive sheet to be exposed to a cooling-heating cycle and this exposure makes the paraffin oil bleed out, leading to contamination of the electronic substrate including the CPU. In addition, the use of the propylene polymer which is a rigid component allows the sheet to lack flexibility. This provides poor proximity of the sheet to the CPU or heatsink and cannot allow the thermal conductivity the sheet originally has to develop effectively. As a result, the heat dissipation of the sheet is poor.
Patent Document 5 proposes that a filler such as alumina (aluminum oxide) is added to a styrene-based thermoplastic elastomer. This addition is intended to improve the thermal conductivity of the resin composition. However, the patent document does not describe use of zinc oxide as a filler to give the composition thermal conductivity, a preferred shape of the filler, or chips generated during production of the composition. Use of alumina having an amorphous or spherical shape, both of which are common alumina structures, provides the occurrence of chips due to alumina removal during strand or sheet cutting and this occurrence results in poor electrical insulation, causing failure of an electronic substrate.
Patent Document 6 proposes a product molded from a thermally conductive resin produced by mixing graphite into a thermoplastic resin. Simply mixing a large amount of graphite increases thermal conductivity greatly but reduces electrical insulation. For this reason, when the molded product is in contact with an electronic substrate, energization creates a short circuit, causing breakage of a semiconductor element. In addition, the product is not preferable because it is a material lacking the flexibility and toughness which a thermally conductive sheet is required to have. In contrast, mixing a small amount of graphite allows the product to maintain electrical insulation but have poor thermal conductivity. For this reason, the product is not sufficient as a thermally conductive material for semiconductor elements which have recently produced more heat.
Patent Document 7 discloses a molded product made from a thermally conductive material comprising a thermoplastic resin and a zinc oxide whisker. However, the patent document does not describe use of a flexible material and a paraffin oil, so the product is a material lacking flexibility and toughness. This lack provides poor proximity of the product to a cooling part and a heating element and cannot allow the thermal conductivity of the product to develop effectively, so the product is not suitable as a thermally conductive sheet.    Patent Document 1: Japanese Patent Laid-Open No. 1-225663    Patent Document 2: Japanese Patent Publication No. 7-51646    Patent Document 3: Japanese Patent Laid-Open No. 2003-277560    Patent Document 4: Japanese Patent Laid-Open No. 2003-49046    Patent Document 5: Japanese Patent Laid-Open No. 2002-206030    Patent Document 6: Japanese Patent Laid-Open No. 62-131033    Patent Document 7: Japanese Patent Laid-Open No. 2006-57064